The present invention may be employed in a digital, continuous tone, or color copier which is capable of producing continuous tone output from digital input, using an analog raster output scanner (ROS). While it is commonly known to use halftoning techniques, error diffusion and pulse-width modulation to digitally represent continuous tone images with a binary ROS, these techniques are unavailable for the control of an analog output ROS. Moreover, reproducing continuous tone images requires precise control of the exposure intensity, for example, the intensity of a laser beam impinging upon a photosensitive surface.
In an analog ROS system employing a laser as the light source, two difficulties are encountered in attempting to accurately control the exposure intensity of the laser light beam. The first is the typical nonlinear characteristic of the acousto-optic modulator (AOM) which modulates the exposure beam intensity in accordance with an input intensity level. Second are the fluctuations in the intensity of the beam produced by the laser, due primarily to warm-up and aging effects.
Heretofore, various techniques have been employed to calibrate or adjust raster input scanners (RISs) and ROSs. Some of the approaches used are illustrated in the following disclosures which may be relevant:
U.S. Pat. No. 4,980,778 PA0 U.S. Pat. No. 4,763,199 PA0 U.S. Pat. No. 4,757,389 PA0 U.S. Pat. No. 4,751,377 PA0 U.S. Pat. No. 4,748,515 PA0 U.S. Pat. No. 4,647,981
Patentee: Wittman PA1 Issued: Dec. 23, 1990 PA1 Patentee: Suzuki PA1 Issued: Aug. 9, 1988 PA1 Patentee: Clark et al. PA1 Issued: Jul. 12, 1988 PA1 Patentee: Ishizaka et al. PA1 Issued: Jun. 14, 1988 PA1 Patentee: Van Daele et al. PA1 Issued: May 31, 1988 PA1 Patentee: Froelich PA1 Issued: Mar. 3, 1987
The relevant portions of the foregoing patents may be briefly summarized as follows:
U.S. Pat. No. 4,980,778 to Wittman discloses a method of correcting non-uniformities in an optical system of a document scanning apparatus. This system includes a plurality of light sensors, each of which have an analog output voltage which varies in proportion to the level, or intensity, of light impinging upon them. The magnitude of the various correction signals are determined during a calibration procedure.
U.S. Pat. No. 4,763,199 to Suzuki teaches a method of reproducing a halftone image, wherein inputted digital image information is converted to an analog form and then compared with a periodic analog pattern signal to obtain pulse-width modulated and binarized image information. A laser driver, whose operation is based upon a pulse-width modulated signal is used to modulate the recording beam.
U.S. Pat. No. 4,757,389 to Clark et al. discloses a calibration method for obtaining correction values to correct for non-uniformities among photosites in an image scanning array associated with electronic image scanning systems. In the system disclosed, light intensity representing an original image is converted into an analog voltage which is then converted into a digital signal using an analog-to-digital (A/D) converter. Accurate calibration is achieved by adjusting image signal outputs of the nonuniform photosites by combining the image signal output of each photosite with a corrective voltage for that particular photosensor.
U.S. Pat. No. 4,751,377 to Ishizaka et al. teaches a light beam scanning recording apparatus, and method, in which a light beam is modulated according to image signals representing an image to be recorded. The method enables the image to be recorded with constant intensity, irrespective of fluctuations in the development conditions of the recording material.
U.S. Pat. No. 4,748,515 to Van Daele et al. discloses a method and apparatus for generating correcting signals to correct a video signal output from an electro-optical document reading apparatus. The method of generating the correction factor signals employs a successive approximation technique.
U.S. Pat. No. 4,647,981 to Froelich teaches a circuit for determining the amount of deviation of intensity level of a scanning beam in a ROS from a desired level during a calibration phase. A correction value is determined as a function of the deviation value, the correction being used during an actual reading or writing phase to normalize the output. A photomultiplier tube (PMT) may be used to collect light reflected from a calibration strip. The output of the PMT is compared to a threshold, stored, and reconverted to an analog voltage which is then used to control the light beam intensity.
The present invention is a method and apparatus for automatically monitoring the output intensity of an analog raster output scanning system. The invention further includes the ability to recognize when the output intensity is not within a predetermined tolerance range, at which time the system will automatically recalibrate to achieve a desired linear transformation from the input image greyscale intensity to the ROS exposure intensity.
In accordance with the present invention, there is provided an analog raster output scanner having a light source for creating a beam of light which is regulated by a modulator responsive to a voltage signal which represents an image intensity level for a spot to be produced on an output medium. The modulated light beam exposes an output medium in proportion to the image intensity level. The raster output scanner is also able to monitor the modulated light beam intensity generated in response to a predetermined intensity level, and compare the actual light beam intensity with a desired intensity to generate a difference signal.
In accordance with another aspect of the present invention, there is provided a method for maintaining a linear output transfer function in an analog raster output scanner. The raster output scanner first programs a look-up table with a set of digital output values, and then uses the look-up table to generate the digital output values in response to image intensity signals received by the raster output scanner. Periodically the scanner passes a predetermined test intensity signal to the look-up table to generate a test digital output value. Subsequently, the intensity of a modulated light beam produced in response to the test digital output value is measured. The scanner then determines whether the measured intensity is within a target range, and if so, continues normal operation. Otherwise, the scanner automatically recalibrates itself to produce a new set of look-up table values and loads the values into the programmabie look-up table.
In accordance with yet another aspect of the present invention, there is provided a method for generating a plurality of distinct intensity values representing a mapping function in an analog raster output scanner. The mapping function, implemented as a look-up table, produces a modulation signal in response to an image intensity signal, and the modulation signal is then used to modulate the intensity of a light beam. The method begins by selecting a test level, i, and generating a modulated light beam in response to the selected test level. Next, the intensity of the modulated light beam is measured and a value indicative of the measured intensity is stored in a memory location indexed in accordance with the selected test level. This process is repeated until all possible values of i have been selected. Subsequently, stored intensity values representative of desired output intensities for each of a series of possible image intensity signals, j, are isolated and stored in the look-up table for use in generating digital output values in response to image intensity signals received by the raster output scanner.